In optical imaging systems it may be necessary to correct for optical aberrations, or phase errors, that blur the image of an object under observation. Typical applications where such correction is required are e.g. ground based telescopes, where the image of an observed object is blurred by the atmosphere, and retinal imaging, where the image of the retina is blurred by the eye's optical aberrations.
One way to correct for such aberrations is to use an adaptive optics (AO) system. Adaptive optics has been applied in different areas of science and industry, e.g. to enhance the capabilities of imaging systems in astronomy, microscopy, and ophthalmology, to enhance signal quality in optical communication systems, and also in laser beam control. In environments where an imaging system is used to observe objects behind a continuously evolving phase curtain (atmosphere, ocular optics, heating effects, etc.), the adaptive optics system can effectively mitigate the effects of this medium to regain the loss of imaging performance.
In systems using adaptive optics, a Shack-Hartmann wavefront sensor may be used to detect an optically aberrated wavefront. A Shack-Hartmann sensor usually makes use of a point source having a known position, a so-called guide star, located somewhere within the field of view to be imaged. By sampling the wavefront of the guide star image, and comparing the acquired samples with known reference samples, it is possible to determine the aberration of a wavefront. Furthermore, it is possible to measure aberrations over a larger field of view by using multiple guide stars.
Once the optical aberrations are known, the AO-system may correct the aberrated wavefront by using one or more wavefront modifying devices, e.g. deformable mirrors or reflective or transmissive phase modulators.
However, with the use of multiple guide stars, it is increasingly important to accurately align the guide star source with various optical elements in the optical system. A system using multiple guide stars thereby becomes more sensitive to misalignment of optical elements, which may for example occur through mechanical movement or through thermal expansion.
In U.S. Pat. No. 6,634,750, a plurality of guide stars is formed by using an array of light-sources to provide a plurality of light beams, each representing a guide star. Each point source in the array provides a collimated light beam which passes through a common lens in order to provide a set of collimated beams at different angles. As the collimated beams are diverging, it is critical that the following optical elements, such as lenses and apertures, are correctly positioned, as any misalignment will lead to subsequent errors in the detection system.
Accordingly, there is a need for a more robust optical system using multiple guide stars and a wavefront sensor to detect optical aberrations.